An error rate monitor is disposed in the receivers associated with a telecommunications transmission link and arranged to monitor errors occurring in messages transmitted over the link from transmitters associated with the link. Typically, the transmitters include transmit buffers in which messages awaiting transmission are stored or queued. The error rate monitor is supposed to take the link out of service (perform a "changeover") if an error rate greater than a predetermined maximum (.lambda..sub.0) persists. While different factors govern the design of a monitor, one known design criterion is that the total length measured (in bits, bytes, octets, etc.) of the unsuccessfully transmitted messages stored in the transmit buffer when changeover occurs (excluding data stored in the transmit buffer prior to the onset of errors and after changeover is declared) should be no more than .OMEGA., when the link is operated at a maximum agreed upon utilization (engineered load) of .rho. or less.
One existing arrangement for an error rate monitor, called a Signaling Unit Error Rate Monitor, or SUERM for short, is used to monitor errors on a signaling link connecting signaling nodes in a SS7 signaling system. A SUERM uses two counters, that maintain counts C.sub.1 and C.sub.2. C.sub.1 (initially 0) is incremented on each errored message. C.sub.2 (initially 0) is incremented for each message. When C.sub.1 exceeds a threshold, T.sub.1, the need for a changeover is declared. When C.sub.2 exceeds a threshold, T.sub.2, C.sub.1 is decremented (to a minimum of zero) and C.sub.2 is reset to zero. When a signaling link loses synchronism (typically during an error burst--the most common error phenomenon), the SUERM enters octet counting mode, wherein messages are modeled as being 16 octets long which are all counted as being errored.
For high speed signaling links, the current SUERM design disadvantageously requires the use of very high quality transmission media, i.e., media in which the background bit error rate is very low. This is because this design for a SUERM cannot distinguish adequately between the situation where many errored messages occur relatively close to each other within the round trip delay of the transmission medium (having the same effect on the total length of the messages stored in the transmit queue as a single errored message) and the situation where the same number of errored messages occur, but are spaced further apart in time, such as where the spacing between successive errored messages is about equal to the round trip delay. In the latter situation, the per error impact on the system is much greater than in the former situation. Because of this inability to distinguish between a relatively benign condition and a much more serious condition, a system which uses the current design must generally be "overdesigned" in temps of higher quality transmission media, thus compensating for the lower effectiveness of the SUERM.
Another arrangement for an error rate monitor is described in U.S. Pat. No. 4,385,383 issued to R. Karchevski on May 24, 1983. In the arrangement described in the patent, a transmission link is monitored for errors during each of a series of fixed time intervals (windows). If one or more errors occur within a window, the count maintained in a counter is incremented by one unit. If no errors occur within the window, the counter is decremented by one unit. When the count in the counter reaches a threshold value, the need for a changeover is declared, or an alarm is sounded. This arrangement is not practical as a SUERM, because it usually results in a situation where an excessive amount of information would have to be stored in the transmit buffer when the need for a changeover is declared. Also, this monitor, which enforces .lambda..sub.0, requires an excessive time period to respond to an error condition that requires a changeover.